Electrical systems of aircraft and other specialized vehicles typically include line-replaceable units (LRUs). LRUs are modular electronics systems, which perform various vehicle operations. LRUs generally have input and output electrical connections and an internal bus, typically a serial bus.
Aircraft LRUs may be categorized according to how critical they are to the aircraft operation, e.g., categorized according to criticality. The least critical LRUs are responsible for operations such as passenger in-flight entertainment, while the most critical LRUs are responsible for aircraft airworthiness, e.g., flight controls. For example, the Aircraft Control Domain (ACD) LRU is a most-critical LRU, the Aviation Information Systems (AIS) LRU is a middle-critical LRU, and the In-Flight Entertainment (AIF) LRU is a least-critical LRU. In commercial aviation, levels of criticality are stratified into Design Assurance Levels (DAL). They range from Level A to Level E. ACD is typically categorized as either Level A or Level B. AIS is typically categorized as Level C or D. Most IFE systems are categorized as Level E. The Table below presents DAL levels.
TABLELevelFailureACatastrophicBHazardousCMajorDMinorENo Effect
Aircraft Communications Addressing and Reporting System (ACARS) is a digital datalink technique and set of standards for transmission of messages between aircraft and ground station and between different intra-aircraft LRUs. An example ACARS standard is the ARINC 429 Specification, available from Rockwell Collins. ACARS may be used for one-way communication or, by using multiple unidirectional systems, two-way communication. ACARS messages typically consist of one or more 32-bit “words”, each of which may include one or more predefined fields that each may contain data values. Specific formats may vary, but in general, each ACARS word includes an 8-bit label (defining the word's data format) and a parity bit (used for error detection), leaving 23 bits available for message content or other data.
ACARS words may include a Source/Destination Identifier (SDI) field, which defines message routing parameters, e.g., by identifying a number of destination LRUs. This information, together with aircraft LRU topology information, determines the LRU(s) to which the message is sent. ACARS messages sent from a ground station to an aircraft may pass through a number of LRUs sequentially, e.g., from a least-critical LRU to a most-critical LRU. The data in the SDI field may be modified as an LRU transfers the message to the next LRU in the sequence in order to indicate the remaining LRUs to which the message is to be sent.
ACARS messages may be used to transfer bulk data, e.g., by employing the bit-oriented Williamsburg/Buckhorn protocol. Such a protocol may begin with a handshake message exchange as follows. The message sender sends a Request to Send (RTS) word, which may include any, or a combination, of the following: a timestamp, a destination code, and a word count. The message receiver responds with a Clear to Send (CTS) word, which also includes a timestamp, the destination code, and the word count. Once the message source receives and confirms the CTS, it may begin the bulk data transfer.
Currently, there is no efficient way to modify an aircraft to handle encrypted communications sent to and from its LRUs, without requiring a costly and time-consuming Federal Aviation Administration (FAA) recertification, e.g., via a Supplemental Type Certificate (STC).